The Art of Passive Dust Control

Conveyor transfer points for cement plants are designed primarily for material throughput and secondarily for fugitive material control. However, dust control and spillage have serious workplace safety consequences that raise the cost of operation from cleanup and equipment fouling. This makes addressing dust and spillage through engineered transfer chute design both critical and practical.

The testing and application of 'passive dust control measures' in a wide range of bulk handling applications using dust curtains and skirting have proven to be an effective control measure for dust. However, this equipment needs to be installed correctly and strategically to maximise results and the return on investment.

When redesigning a dust control enclosure at a conveyor belt transfer point, skirtboard extensions (wear liners, polyurethane skirting, clamps, etc.) to seal the environment and the placement of dust curtains to control airflow are essential. Most engineering firms use the American Conference of Governmental Industrial Hygienists (ACGIH) industrial ventilation handbook design criteria for active dust control (dust machines, dust bags, sprayers, etc.). The design rules for skirtboard extensions are based primarily on lump size and the length necessary for the bulk material to settle down into a stable profile.

By understanding the environment inside the transfer point and how the structural components work together during operation, dust and spillage can be mitigated and the external environment will improve.

Dust dynamics

Air is very compressible and will find the path of least resistance. With current enclosure designs, the air is sped up significantly to flow under or around a single exit dust curtain with narrow slits, resulting in re-entraining the dust particles in the exhaust. Therefore, it is necessary to create recirculation regions inside a transfer point to improve dust settling.

The basic concept is the trajectory of a dust particle can be modelled based on the terminal velocity (Vt) of the dust particle settling in still air and the velocity of the airflow in the transfer point (Vair). The result of these two velocities using the enclosure height (H) as the vertical drop distance indicates the length (L) necessary to settle the dust particle. If the terminal velocity of the particle is very small and the transfer point air speed is relatively large, the settling distance can be quite long.

Using the commonly applied Stoke’s Law, a 10 μm (micrometer) respirable limestone dust particle in an air stream travelling 1 m/s (3.2 ft/s) is predicted to take 75 m (246 ft) to settle by gravity alone. This makes a well-designed enclosure with the proper height, seal, and air control with curtains essential to controlling dust.

Preferred embodiments

When reducing dust emissions, field tests have shown the difference in performance for longer and taller skirtboards of 4800 mm (15 ft) long and 900 mm (3 ft) high compared to 3600 mm (~12 ft) long and 600 mm high is negligible. It was the placement of dust curtains that had the greatest impact on dust settling.

In both transfer point sizes, enclosures with 3 curtains spaced 300 mm (~1 ft) apart from the entrance and exit and one in the centre offered superior performance as compared to one dust curtain at the end.

The best value for the cost of the skirtboard enclosure and its effectiveness is 600 mm high and 3600 mm long, using either the retrofit or mitred discharge chute-to-skirtboard connection.

The junction between the discharge chute and the skirtboards was found to be an important design detail for creating recirculation. Most conveyor engineers and manufacturers use 300 mm high skirtboards, because this height is about the minimum for installing a sealing system and wearliners.

In most of the models, the discharge chute was 200 mm (7.78 in.) narrower than the skirtboards. Making the width of the discharge chute narrower than the width of the skirtboard helps to fold the airflow going into the first curtain, and that encourages the distribution of the airflow toward the top of the enclosure rather than along the surface of the bulk material. Extending the head chute back to the first full troughing idler on the carrying side and using 2 curtains plus sealing the area between the top and bottom runs is critical in reducing induced airflow.

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